1. Field of the Invention
The present invention relates to an image forming apparatus employing an electrophotographic recording method such as a laser printer, a copying machine, and a facsimile device.
2. Description of the Related Art
As a conventional electrophotographic image forming apparatus, a color image forming apparatus employing an inline method, in which image forming units of respective colors are arranged side by side, is known. In this color image forming apparatus, a plurality of image forming units is arranged side by side. Each of the image forming units includes an electrophotographic photosensitive member (hereinafter, referred to as a photosensitive drum) and process units that work on the photosensitive drum, such as a charging unit, a developing member, and a cleaning unit. Images are sequentially transferred onto an intermediate transfer belt or a transfer material or the like on a conveyer belt facing the plurality of image forming units.
In such a color image forming apparatus, color shift may occur when the images overlap with each other due to a difference in mechanical accuracy and the like among the image forming units corresponding to respective colors (for example, yellow (Y), magenta (M), cyan (C), and black (K)). In particular, in a configuration in which a laser scanner and the photosensitive drum are independently provided for each of the image forming units corresponding to respective colors, the positional relationship between the laser scanner and the photosensitive drum differs for each color. Thus, the color shift occurs due to a failure to synchronize laser scanned positions on the photosensitive drums.
In view of the above, color shift correction control is performed in the color image forming apparatus to correct the color shift. Japanese Patent Application Laid-Open No. 2012-032777 discusses a circuit configuration of accurately correcting the color shift occurring when images of a plurality of colors overlap with each other. More specifically, a common current detection circuit is provided for independent high voltage power sources that supply current to the image forming units corresponding to the respective colors.
However, in the circuit configuration discussed in Japanese Patent Application Laid-Open No. 2012-032777, the independent high voltage power sources, connected to each other via a common current detection circuit, may affect each other to cause an image failure.
More specifically, when a second independent high voltage power source is turned on while a first independent high voltage power source is in an ON state, charge current generated in a second high voltage power source circuit instantaneously flows into the current detection circuit, and thus a potential between input and output terminals changes due to a resistor component in the current detection circuit. The potential change affects a first high voltage power source circuit, and thus a value of voltage output from the first high voltage power source circuit changes.
Thus, charging voltage for the color K changes when a mode is switched to a full color mode when successive printing in a monochrome mode is in process, for example. More specifically, when charging voltage for the colors Y, M, and C is applied while the charging voltage for the color K is being applied, charge current supplied to power source circuits for the colors Y, M, and C instantaneously flows into the current detection circuit to change the charging voltage for the color K. As a result, an image failure such as a traverse line occurs in an image due to transfer of toner onto a portion of the photosensitive drum for the color K where a charging output is small.
Such a problem occurs not only in the circuit configuration in which the independent high voltage power sources are connected to each other via the common current detection circuit, but also in any circuit configuration in which independent high voltage power sources are grounded via a common resistor component. Furthermore, the problem may also occur in a configuration in which independent high voltage power sources are arranged on a common substrate that is not grounded to a frame ground (a casing) of an apparatus main body.